Chromatic dispersion compensating apparatus

ABSTRACT

In order to compensate for chromatic dispersion ad dispersion slope over an entire wavelength band of the optical signal, the wavelength band is split into a plurality of bands, and chromatic dispersion compensation is made to make chromatic dispersion in a central wavelength of each of the bands zero.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a chromatic dispersioncompensating apparatus for further increasing the capacity, the speed,and the distance of an optical communications system hereafter.

[0003] 2. Description of the Related Art

[0004] With the sharply growing network use in recent years, the demandfor further increasing the capacity of a network has been rising.Currently, a wavelength multiplexing (WDM) optical transmission systemon a basis of a transmission rate of 10 Gb/s per channel has been putinto practical use. Hereafter, a further increase in the capacity isexpected to be required, and an ultrahigh-speed transmission system of40 Gb/s or faster per channel is demanded from the viewpoints offrequency use efficiency and cost. In an ultrahigh-speed transmissionsystem, wavelength degradation caused by dispersion of a transmissionline must be compensated with high accuracy.

[0005] In an optical transmission system having a transmission rate of10 Gb/s or faster, a chromatic dispersion tolerance is very small. Forexample, the chromatic dispersion tolerance of a 40-Gb/s NRZ system isequal or smaller than 100 ps/nm. In the meantime, for a terrestricaloptical transmission system, span length are not always uniform. In thecase of a system using a 1.3-μm zero dispersion single mode fiber (SMF)of approximately 17 ps/nm/km, a chromatic dispersion tolerance isexceeded if the length differs only several kms. However, in an opticalfiber network possessed by a communications carrier, most span lengthand chromatic dispersion values are not accurately grasped at present.Additionally, since a chromatic dispersion value changes with timedepending on a fiber temperature, stress, etc., a dispersioncompensation amount for each span must be adjusted not only at the startof system operations but also in system use while strictly monitoring achromatic dispersion amount. For example, a temperature change of 100°C. occurs on a 500-km DSF (Dispersion Shifted Fiber) transmission line,its chromatic dispersion change amount becomes approximately 105 ps/nmthat is almost equal to a chromatic dispersion tolerance of a 40-Gb/sNRZ signal.

(chromatic dispersion change amount)=(temperature dependency of a zerodispersion wavelength)×(temperature change amount of a transmissionline)×(dispersion slope of the transmission line)×(transmissiondistance)

=0.03 (nm/° C.)×100 (° C.)×0.07 (ps/nm ² /km)×500 (km)

=105 ps/nm

[0006] Therefore automatic dispersion compensation is essential for asystem using not only an SMF transmission line, but also a 1.55-μm zerodispersion shifted fiber (DSF) or an NZ-DSF transmission line.

[0007] Furthermore, when a wavelength-division multiplexed(WDM) signalis transmitted, a dispersion slope as well as chromatic dispersion mustbe considered.

[0008]FIG. 1 exemplifies the configuration of a WDM transmission system.FIG. 2 shows a change in a chromatic dispersion amount of a transmissionline due to various change factors.

[0009] In the configuration shown in FIG. 1, optical signals ofrespective wavelengths are transmitted from optical transmitters #1 to#n of a transmitting end station device, and coupled by an opticalmultiplexer. The multiplexed optical signal is amplified and output byan optical post-amplifier. When the process for amplifying the opticalsignal is performed by the optical post-amplifier, dispersioncompensation is made for the optical signal by a transmission dispersioncompensator in the transmitter, whose dispersion compensation amount isfixed or variable. The optical signal which propagates over a fibertransmission line is amplified so that a transmission line loss iscompensated by an optical inline amplifier, which exists partway of thefiber transmission line. Additionally, chromatic dispersion that theoptical signal undergoes as a result of propagating over thetransmission line is compensated by an inline dispersion compensator,when the amplification is made by the optical inline amplifier. Thedispersion compensation amount of the inline dispersion compensator maybe fixed or variable. Furthermore, the optical signal is propagated overthe fiber transmission line via an inline amplifier, and input to anoptical receiver.

[0010] In the optical receiver, the propagated optical signal isamplified so that its attenuation is compensated. At this time,dispersion compensation also at the receiver side is made by a receptiondispersion compensator in the receiver. Then, the propagated opticalsignal is split into respective wavelengths by an optical demultiplexer.For example, variable dispersion compensators remove residual dispersionfrom the optical signals of the demultiplexed wavelengths, and thesignals are received by optical receivers #1 to #n. Here, the reason whythe variable dispersion compensators are enclosed by brackets is thatthey are not always necessary. Whether or not the variable dispersioncompensator are included can be determined by a designer depending onthe details of a design. If a constituent element is enclosed bybrackets also in the subsequent configuration drawings, it means thatthe constituent element is not always required to be included at thediscretion of a designer.

[0011] For a temperature change in the chromatic dispersion of anoptical signal, a chromatic dispersion characteristic (a) shifts to (c)according to a temperature change (approximately 0.03 nm/° C.) in a zerodispersion wavelength as shown in FIG. 2. In this case, a dispersionslope does not change. Additionally, if a transmission distance isdifferent, the chromatic dispersion characteristic (a) changes to (b).In this case, also the dispersion slope changes with the dispersionamount. For an actual transmission line fiber (and a dispersioncompensation fiber (DCF)), the chromatic dispersion value ((a)→(c)), andthe dispersion slope ((a)→(d)) have variations due to a problem of afiber manufacturing ability, even if the length of a transmission lineis the same.

[0012] As a means for compensating for chromatic dispersion and adispersion slope, the following methods are considered.

[0013] (a) Implementing a broadband variable dispersion compensator thatcan independently vary a chromatic dispersion amount and a dispersionslope amount, and making dispersion compensation simultaneously forsignals of all of wavelengths.

[0014] (b) Independently arranging a broadband variable dispersioncompensator that can vary a chromatic dispersion amount, and a broadbandvariable dispersion slope compensator that can vary a dispersion slopeamount, and making dispersion compensation collectively for signals ofall of wavelengths.

[0015] (c) Independently arranging a broadband variable dispersioncompensator that can vary a chromatic dispersion amount, and a fixeddispersion slope compensator whose dispersion slope amount compensates aslope amount of a transmission line, and making dispersion compensationsimultaneously for signals of all of wavelengths.

[0016] (d) Individually arranging for each channel a variable dispersioncompensator that can vary a chromatic dispersion amount, and makingdispersion compensation.

[0017] The most important point in the methods (a) to (d) is thepracticability of a variable dispersion compensator.

[0018]FIG. 3 shows a VIPA (Virtually Imaged Phased Array) as an exampleof a variable dispersion compensator. As documents about a VIPA, pleasesee M. Sirasaki et al., “Variable Dispersion Compensator Using theVirtually Imaged Phased Array (VIPA) for 40-Gbit/s WDM TransmissionSystem”, ECOC 2000, Post-deadline paper 2.3., etc.

[0019] In a dispersion compensator using a VIPA, a dispersioncompensation amount can be successively changed in a range from −800ps/nm to +800 ps/nm by moving a three-dimensional mirror in thedirection of an x axis.

[0020]FIG. 4 shows a transmittance characteristic and a group delaycharacteristic of a VIPA variable dispersion compensator.

[0021] The transmittance characteristic shown in an upper portion ofthis figure exhibits a periodical wavelength dependence of transmittancein a VIPA. Accordingly, a design must be made so that optical signals ofrespective wavelengths of wavelength multiplexed light (WDM light) passthrough a high portion of the transmittance, namely, a transmittancewindow. Additionally, the figure of the group delay represents that thegroup delay is periodically given to an optical signal. It is provedfrom this figure that the slope of the group delay in a portion wherethe transmittance window is opened is decreasing on the right, andnegative dispersion is given to an optical signal that passes throughthe window.

[0022] For example, a VIPA is designed to have a cyclic structure wherea transmission characteristic has a frequency interval (free spectralrange: FSR) of 200 GHz (wavelength interval is 1.6 nm), and advantageousto simultaneously compensate for a WDM signal. However, the VIPA cannotcompensate for a dispersion slope. A system implemented by combining aVIPA dispersion compensator and a dispersion compensation fiber in orderto collectively compensate for the dispersion compensation and thedispersion slope is proposed by Japanese Patent Application No.2000-238349.

[0023]FIG. 5 shows the group delay characteristic of a VIPA variabledispersion compensator in a channel passband.

[0024] In a variable dispersion compensator using a VIPA shown in anupper portion of FIG. 5, changes in the slope of a group delay shown ina lower portion of FIG. 5 are obtained by moving a three-dimensionalmirror in the direction of an x axis. Dispersion is obtained bydifferentiation of wavelengths of a group delay. Therefore, simultaneousdispersion compensation can be varied and made depending on need for allof channel bands by moving the three-dimensional mirror.

[0025]FIG. 6 exemplifies the configuration of an optical receiveraccording to a conventional technique.

[0026] In the configuration example shown in this figure, a DCF whosedispersion slope amount (a dispersion slope of a transmission line)arranged to compensate for a dispersion slope of the transmission line.Furthermore, chromatic dispersion caused by the transmission line andthe DCF is collectively compensated by using a VIPA variable dispersioncompensator. As shown in FIG. 4, the VIPA has the periodical structureof 200 GHz intervals in order to secure a transmission band. In acurrent dense WDM transmission system, 100 GHz channelspacing(wavelength interval of 0.8 nm) is demand. Accordingly, in FIG.6, a received signal of 100-GHz spacings is separated into even- andodd-numbered channels of 200-GHz intervals by using an interleaver, anddispersion compensation is simultaneously made by arranging VIPAdispersion compensators respectively for the even- and odd-numberedchannels. As shown in FIG. 7, a transmittance window of the interleveris opened in predetermined cycles (200 GHz in this case) A solid lineshown in this figure is a window for extracting odd-numbered channels,whereas a dotted line shown in this figure is a window for extractingeven-numbered channels. As described above, the interleaver alternatelysamples a wavelength multiplexed optical signal, and separates theoptical signal into even- and odd-numbered channels, so that the channelintervals of the optical signal after being separated are widened.

[0027] However, this configuration has a problem stemming from thewavelength dependency of a dispersion slope of a transmission line and aDCF, leading to a difficulty in simultaneous dispersion compensation.

[0028]FIG. 8 shows a typical example of a dispersion characteristic on afiber transmission line.

[0029] Mainly on a DCF, a dispersion curve derived from the wavelengthdependency of a dispersion slope occurs due to a manufacturing problem(however, an almost linear dispersion characteristic is possessed in atransmission fiber). Accordingly, residual dispersion derived from thewavelength dependency of a dispersion slope occurs on a transmissionline and a DCF. In a long-haul transmission, this residual dispersionbecomes a value that exceeds the dispersion tolerance of a 40-Gb/ssignal. Therefore, simultaneous compensation is difficult with theconfiguration of FIG. 6 itself.

[0030] Also an implementation of a dispersion monitor for detecting achromatic dispersion amount (and a slope amount), which a transmissionline undergoes, is important to realize an automatic dispersioncompensating system.

[0031] As an example of a dispersion monitor method, there is a methodusing the intensity of a particular frequency component within areceived baseband signal.

[0032]FIG. 9 shows a result of detecting the intensity of a 40-GHzcomponent within a received baseband signal of a 40-Gb/s NRZ signal.

[0033] Source: Y. Akiyama et al., “Automatic Dispersion Equalization in40 Gbit/s Transmission by Seamless-switching between Multiple SignalWavelengths”, ECOC '99, pp. 1-150-151

[0034] As is known from a calculation result shown on the left side, theintensity of a 40-GHz component varies with a chromatic dispersionamount, and becomes zero when the dispersion amount is zero. In anexperimental result of a 100-km DSF transmission on the right side, thedispersion amount of a transmission line varies with a wavelength.Therefore, the intensity characteristic of a 40-GHz component isobtained in a similar manner as in the calculation result. A zerodispersion wavelength of the transmission line varies with a change inthe temperature of the transmission line by approximately 0.03 nm/° C.However, it can be verified that also the minimum point of an intensitymonitor of the 40-GHz component varies with that change. It is knownthat the intensity of a B Hz component is available as a chromaticdispersion monitor for a B b/s modulation signal also with othermodulation methods. It is known, for example, when chromatic dispersionis zero, the intensity of a B Hz component becomes a maximum for an RZsignal, and becomes a minimum for an OTDM signal (Japanese PatentApplication No. Hei 9-224056).

[0035] As another means, a method monitoring a bit error ratecharacteristic or a Q value, which is detected by each optical receiver,is considered.

[0036] To implement a low-cost dispersion monitor in a wavelengthmultiplexing system, a method arranging a dispersion monitor isimportant. For example, in the case of (a) or (b) shown in FIG. 2, ifchromatic dispersion amounts of at least two signals such as signals ofwavelengths at both ends of a signal wavelength band can be detected, adispersion slope can be learned by extrapolation, and a chromaticdispersion amount of a different signal wavelength can be detected.

[0037] Additionally, in the case of (c), the dispersion slope amount ofthe transmission line does not vary with a temperature change.Therefore, if a chromatic dispersion amount of at least one signal suchas a central wavelength signal, etc. of a signal wavelength band can bedetected, a chromatic dispersion amount of a different signal wavelengthcan be detected from the chromatic dispersion amount and the knowndispersion slope amount.

[0038] Also in the case of (d), a chromatic dispersion amount of adifferent signal wavelength can be detected by extrapolation, if achromatic dispersion value of at least one wavelength signal can bedetected when a dispersion slope amount (or the length of a transmissionline) is known, or if chromatic dispersion values of at least twowavelength signals can be detected when the dispersion slope amount isunknown.

[0039] The above described problems of conventional techniques aresummarized below.

[0040] In an optical transmission system having a transmission rate of10 Gb/s or faster, a chromatic dispersion tolerance is very small. Forexample, the chromatic dispersion tolerance of a 40-Gb/s NRZ system isequal to approximately 100 ps/nm or smaller. In the meantime, for thechromatic dispersion of a transmission line, the following changefactors exist. If a wavelength-division multiplexed(WDM) signal istransmitted, not only chromatic dispersion but also a dispersion slopemust be considered.

[0041] (1) Difference in the Length of a Transmission Line

[0042] For a terrestrical optical transmission system, lengths of itsspan length are not always uniform. In the case of a system using a1.3-μm zero dispersion single-mode fiber (SMF) of approximately 17ps/nm/km, a chromatic dispersion tolerance is exceeded if the length isdifferent by only several kilometers. However, in an optical fibernetwork possessed by a communications carrier, most span length andchromatic dispersion values are not accurately grasped at present. Asshown in FIG. 2, the chromatic dispersion characteristic (a) changes to(b) if a transmission distance is different. In this case, also thedispersion slope as well as the dispersion amount changes.

[0043] (2) Incompleteness of a Slope Compensation Ratio of a DispersionCompensation Fiber (DCF)

[0044] To make dispersion compensation and dispersion slope compensationsimultaneously for a wavelength multiplexed signal, a dispersioncompensating fiber (DCF) having a dispersion slope rate (dispersionslope coefficient/chromatic dispersion coefficient) that matches atransmission line must be used. However, especially for an NZDSF fiber(such as Enhanced LEAF, TrueWave Plus, TrueWave Classic, etc.) having asmall chromatic dispersion coefficient, a DCF that can be manufacturedis only a DCF whose slope compensation ratio is as low as 50 to 60percent.

[0045]FIG. 10 shows fluctuations of chromatic dispersion on atransmission line due to dispersion slope variations.

[0046] In this figure, to compensate for a dispersion slopecharacteristic (a) of the transmission line by 100 percent, it is idealthat a DCF matches a characteristic (a)′ of its reverse sign. Actually,however, a slope compensation ratio as high as (a)′ cannot be obtained,and the ratio becomes like (b). As a result, residual dispersionindicated by (c) occurs on the transmission line and the DCF.

[0047] (3) Manufacturing Variations of a Chromatic DispersionCoefficient and a Dispersion Slope Coefficient of a Transmission LineFiber and a Dispersion Compensating Fiber (DCF)

[0048] Since a chromatic dispersion coefficient (chromatic dispersionamount per unit length. The unit is ps/nm/km), and a dispersion slopecoefficient (chromatic dispersion slope per unit length. The unit isps/nm²/km) of a transmission line and a dispersion compensating fiber(DCF) reach the limits of manufacturing accuracy, they have relativelylarge variations. Therefore, as shown in FIG. 2, the chromaticdispersion amount (the unit is ps/nm. (a)→(c), −(a)→(b)′), and thedispersion slope amount (the unit is ps/nm², ((a)→(d), −(a)→(d)′)) vary,even if the lengths of the transmission line and the DCF are the same.

[0049] (4) Temperature Change in a Zero Dispersion Wavelength of a Fiber

[0050] Since the zero dispersion wavelength of a transmission line fiberchanges with time depending on a temperature, a dispersion compensationamount for each span must be suitably set while strictly monitoring achromatic dispersion amount not only at the start of system operations,but also in system use.

[0051] For example, if a temperature change of 100° C. occurs on a600-km transmission line, a chromatic dispersion change amount becomesapproximately 108 ps/nm, which is almost equal to the chromaticdispersion tolerance of a 40-Gb/s NRZ signal.

(chromatic dispersion change amount)=(temperature dependency of zerodispersion wavelength)×(temperature change amount of transmissionline)×(dispersion slope of transmission line)×(transmission distance)

=0.03 (nm/° C.)×100° C.×0.06 (ps/nm ² /km)×600 (km)

=108 ps/nm

[0052] In FIG. 2, the chromatic dispersion characteristic (a) changes to(c) due to a temperature change (approximately 0.03 nm/° C.) of the zerodispersion wavelength. In this case, the dispersion slope does not vary.

[0053] (5) Influence of the Wavelength Dependency of a Transmission LineFiber and a DCF

[0054] As shown in FIG. 8, a dispersion curve derived from thewavelength dependency of a dispersion slope occurs due to a problem ofdesign principle also on a transmission line, mainly on a DCF (thetransmission fiber has an almost linear dispersion characteristic).Accordingly, residual dispersion derived from the wavelength dependencyof the dispersion slope occurs on the transmission line and the DCF. Ina long-haul transmission, this residual dispersion becomes a large valuethat exceeds the dispersion tolerance of a 40-Gb/s signal. This becomesa serious problem when dispersion compensation is made collectively forall of channels.

[0055] Measures according to a known technique is as follows. A variabledispersion compensator must be applied to cope with time-varyingchromatic dispersion fluctuations in (4). As an example of the variabledispersion compensator, the VIPA shown in FIG. 3 exists. As a methodarranging a variable dispersion compensator, there are a method makingcompensation simultaneously for all of channels by also comprising aslope compensation function, a method making compensation simultaneouslyfor all of channels by combining with a variable or fixed dispersionslope compensator, or a method arranging a variable dispersioncompensator for each channel (see Japanese Patent Application No.2000-238349) is considered.

SUMMARY OF THE INVENTION

[0056] An object of the present invention is to provide a chromaticdispersion compensating apparatus that can minimize residual dispersionas much as possible at possibly lowest cost, and its arrangementconfiguration.

[0057] A first chromatic dispersion compensating apparatus according tothe present invention, in a wavelength multiplexed optical transmissionsystem, comprises: a band splitting unit splitting a wavelengthmultiplexed optical signal into a plurality of wavelength bands; and afixed dispersion compensating unit making compensation for residualdispersion for split wavelength multiplexed optical signals.

[0058] A second chromatic dispersion compensating apparatus according tothe present invention, in a wavelength multiplexed optical transmissionsystem, comprises: a band spitting unit splitting a wavelengthmultiplexed optical signal into a plurality of wavelength bands; and avariable dispersion compensating unit making compensation simultaneouslyfor split wavelength multiplexed optical signals.

[0059] A third chromatic dispersion compensating apparatus according tothe present invention, in a wavelength multiplexed optical transmissionsystem, comprises: a variable dispersion compensating unit makingdispersion compensation simultaneously for a whole or part of awavelength multiplexed optical signal; an optical splitting unitsplitting the wavelength multiplexed optical signal; and a fixeddispersion compensating unit making compensation for residual dispersionof an optical signal of each split channel.

[0060] A fourth chromatic dispersion compensating apparatus, in awavelength multiplexed optical transmission system, comprises: a bandsplitting unit splitting a wavelength multiplexed optical signal into aplurality of wavelength bands; and a dispersion compensating unitreducing a residual dispersion difference between bands for respectivesplit wavelength bands.

[0061] A fifth chromatic dispersion compensating apparatus, in amulti-span optical transmission system, comprises a dispersioncompensating unit making 105- to 120-percent over-compensation for achromatic dispersion amount of each span in each inline amplifier afterspan.

[0062] According to the present invention, an efficient and effectivechromatic dispersion compensating apparatus and a method thereof can beprovided at low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

[0063]FIG. 1 exemplifies the configuration of a wavelength multiplexingtransmission system;

[0064]FIG. 2 shows changes in a chromatic dispersion amount of atransmission line due to various change factors;

[0065]FIG. 3 shows a VIPA (Virtually Imaged Phased Array) as an exampleof a variable dispersion compensator;

[0066]FIG. 4 shows a transmittance characteristic and a group delaycharacteristic of a VIPA variable dispersion compensator;

[0067]FIG. 5 shows a group delay characteristic of a VIPA variabledispersion compensator;

[0068]FIG. 6 exemplifies the configuration of an optical receiver of aconventional technique;

[0069]FIG. 7 explains the operations of an interleaver;

[0070]FIG. 8 shows a typical example of a dispersion characteristic on afiber transmission line;

[0071]FIG. 9 shows a result of detecting the intensity of a 40-GHzcomponent within a receiving baseband signal of a 40-Gb/s NRZ signal;

[0072]FIG. 10 shows fluctuations of chromatic dispersion on atransmission line due to dispersion slope variations;

[0073]FIG. 11 explains the principle of preferred embodiments accordingto the present invention;

[0074]FIG. 12 exemplifies a first configuration implementing a preferredembodiment according to the present invention;

[0075]FIG. 13 exemplifies a second configuration of the preferredembodiment according to the present invention;

[0076]FIG. 14 exemplifies a third configuration of the preferredembodiment according to the present invention;

[0077]FIG. 15 exemplifies a fourth configuration of the preferredembodiment according to the present invention;

[0078]FIG. 16 exemplifies a fifth configuration of the preferredembodiment according to the present invention;

[0079]FIG. 17 exemplifies a sixth configuration of the preferredembodiment according to the present invention;

[0080]FIG. 18 explains a chromatic dispersion compensation methodaccording to another preferred embodiment of the present invention;

[0081]FIG. 19 exemplifies a seventh configuration according to anotherpreferred embodiment of the present invention;

[0082]FIG. 20 exemplifies an eighth configuration of the preferredembodiment according to the present invention;

[0083]FIG. 21 exemplifies a ninth configuration of the preferredembodiment according to the present invention;

[0084]FIG. 22 exemplifies a tenth configuration of the preferredembodiment according to the present invention;

[0085]FIG. 23 exemplifies an eleventh configuration of the preferredembodiment according to the present invention;

[0086]FIG. 24 exemplifies a twelfth configuration of the preferredembodiment according to the present invention;

[0087]FIGS. 25A and 25B show the principle configurations in the casewhere a preferred embodiment according to the present invention isapplied to an inline amplifier;

[0088]FIG. 26 explains the principle of a dispersion compensation methodaccording to a preferred embodiment of the present invention (No. 1);

[0089]FIG. 27 explains the principle of a dispersion compensation methodaccording to a preferred embodiment of the present invention (No. 2);

[0090]FIGS. 28A and 28B show the configurations in the case where fixeddispersion compensators for respective wavelength bands are replaced byvariable dispersion compensators in correspondence with FIG. 25;

[0091]FIGS. 29A and 29B exemplify the configurations of a specificinline amplifier in the case where wavelength band split compensation ismade (No. 1);

[0092]FIGS. 30A and 30B exemplify the configurations of a specificinline amplifier in the case where wavelength band split compensation ismade (No. 2);

[0093]FIGS. 31A and 31B exemplify the configurations of a specificinline amplifier in the case where wavelength band split compensation ismade (No. 3);

[0094]FIGS. 32A and 32B exemplify the configurations of a specificinline amplifier in the case where wavelength band split compensation ismade (No. 4);

[0095]FIGS. 33A and 33B exemplify the configurations of a specificinline amplifier in the case where wavelength band split compensation ismade (No. 5);

[0096]FIG. 34 exemplifies the configuration of a system using the inlineamplifiers that make the band split compensation shown in FIGS. 29 to 33(No. 1);

[0097]FIG. 35 exemplifies the configuration of a system using the inlineamplifiers that make the band split compensation shown in FIGS. 29 to 33(No. 2);

[0098]FIG. 36 exemplifies the configuration of a system using the inlineamplifiers that make the band split compensation shown in FIGS. 29 to 33(No. 3);

[0099]FIGS. 37A to 37C show a Q penalty for residual dispersion in thecase of an inline dispersion compensation ratio D_(DCL)=100 percent and114 percent in a 600-km SMF transmission;

[0100]FIG. 38 shows a Q penalty characteristic against an inlineresidual dispersion amount in the case where residual dispersion is madezero by adjusting a dispersion compensation amount at a receiver side ineach case in a 600-km SMF transmission;

[0101]FIG. 39 exemplifies a first configuration corresponding to apreferred embodiment for optimizing an inline dispersion compensationamount;

[0102]FIG. 40 shows a further specific example of the configurationshown in FIG. 39; and

[0103]FIG. 41 exemplifies the configuration implemented by combiningband split dispersion compensation and inline over-compensation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0104] In preferred embodiments according to the present invention,dispersion compensation is made by splitting a wavelength band intopredetermined bands, if residual dispersion derived from the wavelengthdependency of a dispersion slope of a transmission line and a DCF is toolarge to be ignored.

[0105]FIG. 11 explains the principle of preferred embodiments accordingto the present invention.

[0106] For example, in the case of a residual dispersion characteristicof (a) a transmission line and (b) a DCF1 (for simultaneous compensationfor all of channels) shown in FIG. 11, a wavelength multiplexed signalis demultiplexed into a plurality of wavelength bands (four bands inthis figure), and a dispersion compensation amount is finely adjusted byarranging (c) a fixed dispersion compensator (DCF) or a variabledispersion compensator (VDC) for each of the wavelength bands, so thatthe residual dispersion of all of the channels can be reduced to a smallvalue (d). In this figure, chromatic dispersion of a predeterminedamount is given to all of the channels within the wavelength band sothat 100-percent dispersion compensation is made for a centralwavelength of each of the wavelength bands. If fine adjustment is madewith a fixed dispersion compensator, a fixed dispersion compensationamount must be determined by measuring the dispersion characteristic ofa transmission line beforehand (or grasping a dispersion shift amount asa characteristic specific to a fiber). However, if the dispersion of thetransmission line varies with time due to a temperature change, thechromatic dispersion amounts of all of the channels vary in the samedirection. Therefore, the compensation state can be maintained byvarying variable dispersion compensators concurrently used.

[0107] All of the following configurations show examples in the casewhere a wavelength interval of a transmitted wavelength multiplexedsignal is 100 GHz (approximately 0.8 nm). Additionally, only aconfiguration example of a dispersion compensator within an opticalreceiver is shown. However, a similar configuration can be made also inthe case where a dispersion compensator is arranged within an inlineamplifier or a transmitter side station.

[0108]FIG. 12 exemplifies a first configuration for implementing apreferred embodiment according to the present invention.

[0109] In this configuration example, after dispersion compensation ismade simultaneously for all of channels by a DCF1 and a variabledispersion compensator for 100-GHz intervals, wavelength intervals areseparated into 200-GHz (approximately 1.6 nm) intervals by aninterleaver. Then, the wavelength bands are respectively split into nwavelength bands by band split filters, and fine adjustment is made by afixed dispersion compensator in each of the wavelength bands.

[0110] Firstly, a wavelength multiplexed optical signal propagated overa fiber transmission line 10 is amplified by an optical pre-amplifier11, which is a receiver shown in FIG. 12, and at the same time, itsdispersion slope is compensated by the DCF1. Then, the optical signal of100-GHz intervals is input to a variable dispersion compensator 12,which makes dispersion compensation collectively for all of wavelengths.Next, the optical signal of 100-GHz intervals is separated into even-and odd-numbered channels by an interleaver 13, so that the opticalsignal is converted into optical signals of 200-GHz intervals. Theoptical signals are respectively input to band split filters 14-1 and14-2, which split their wavelength bands into n bands. After residualdispersions in the respective bands are compensated by a DCF for a firstorder dispersion fine adjustment 15, the signals are demultiplexed intooptical signals of the respective channels, and received by opticalreceivers #1 to #40 respectively.

[0111] Here, the number of the optical receivers is 40. This is becauseFIG. 12 assumes the number of multiplexed wavelengths to be 40. However,the number of multiplexed wavelengths is not limited to this value.Rather, optical receivers the number of which is according to the numberof multiplexed wavelengths must be arranged. This is similar also in theexplanations of the following configuration examples.

[0112] Here, optical signals input to the band split filter 14-1 aresignals of odd-number channels, whereas optical signals input to theband split filter 14-2 are signals of even-numbered channels. Band splitfilters are already commercialized and sold by companies such as JDSUniphase, OPlink, Dicon, Avanex, HD Fiber Systems, Chorum, etc.

[0113]FIG. 13 exemplifies a second configuration of the preferredembodiment according to the present invention.

[0114] In contrast to FIG. 12, FIG. 13 shows the configuration wherevariable dispersion compensators for 200-GHz intervals are arrangedafter wavelength intervals are separated into 200-GHz intervals.

[0115] An optical signal propagated over a fiber transmission line 10 isamplified by an optical pre-amplifier 11, and at the same time, itsdispersion slope is compensated by a DCF1. The optical signal is theninput to an interleaver 13, which samples the wavelength multiplexedoptical signal of 100-GHz intervals as optical signals of 200-GHzintervals, and transmits odd-numbered and even-numbered channelsrespectively to variable dispersion compensators for 200-GHz intervals12-1 and 12-2. The variable dispersion compensators 12-1 and 12-2respectively perform a chromatic dispersion compensation process for theinput optical signals, and input the signals to band split filters 14-1and 14-2. The band split filters 14-1 and 14-2 split the sampled opticalsignals into n bands. Then, fine adjustment for a first order dispersionis made by DCFs15 for the respective bands. Optical signals of therespective bands for which the fine adjustment is made are demultiplexedinto optical signals of the respective channels by optical DEMUXs 16,and received by optical receivers #1 to #40.

[0116]FIG. 14 exemplifies a third configuration of the preferredembodiment according to the present invention.

[0117] In contrast to FIG. 13, FIG. 14 shows the configuration wherewavelength intervals are separated into 400-GHz (approximately 3.2 nm)intervals by using two-stage interleavers, optical signals are thensplit into n wavelength bands by band split filters, and fine adjustmentis made for the respective wavelength bands by fixed dispersioncompensators. The band split filters have a guard band (wavelength rangewith no transmittance) in a split wavelength position because of itscharacteristic. By widening a wavelength interval of a signal lightwavelength by an interleaver, a requirement for the band split filter isrelaxed (the number of channels that cannot be transmitted becomes zeroor is reduced even if a guard band is wide). Also a configuration wherethe number of stages of interleavers is further increased to still morewiden a wavelength interval may be implemented.

[0118] An optical signal propagated over a fiber transmission line 10 isamplified by an optical pre-amplifier, and at the same time, itsdispersion slope is compensated by a DCF1. The optical signal is thenseparated into even- and odd-numbered channels by an interleaver 13. Asa result, the wavelength intervals of the optical signals change from100 to 200 GHz. Then, chromatic dispersions of the optical signals of200-GHz intervals are compensated by variable dispersion compensators12-1 and 12-2. Interleavers 20-1 and 20-2 further sample the opticalsignals of 200-GHz intervals to optical signals of 400-GHz intervals,which are respectively input to band split filters 14-1 to 14-4. Theband split filters 14-1 to 14-4 split the bands of the input signalsinto n bands. Then, DCFs15 make fine adjustment for a first orderdispersion compensation for the respective bands, and the signals areinput to optical DEMUXs 16. Optical signals that are demultiplexed intorespective channels by the optical DEMUXs 16 are received by opticalreceivers #1 to #40 respectively. Widening a wavelength interval of anoptical signal before its band is split as described above reduces thepossibility that an optical signal comes at a band boundary when theband is split. Accordingly, this is effective in a sense that an opticalsignal erased by band splitting is eliminated.

[0119]FIG. 15 exemplifies a fourth configuration of the preferredembodiment according to the present invention.

[0120] In contrast to FIG. 14, FIG. 15 shows the configuration whereafter signals of the same wavelength band are combined by interleaverssubsequent to band split filters, fine adjustment is made by fixeddispersion compensators. With this configuration, the number of fixeddispersion compensators for fine adjustment can be reduced.

[0121] A wavelength multiplexed signal propagated over a fibertransmission line 10 is amplified by an optical pre-amplifier 11, and atthe same time, its dispersion slope is compensated by a DCF1. Then, theoptical signal is sampled by an interleaver 13, and optical signalshaving channel intervals (200 GHz) that are double the channel intervals(100 GHz) of the original wavelength multiplexed optical signal arerespectively input to variable dispersion compensators 12-1 and 12-2.After dispersion compensation is made for the optical signals by thevariable dispersion compensators 12-1 and 12-2, the signals are furthersampled by interleavers 20-1 and 20-2. The signals change to thosehaving channel intervals (400 GHz) that are four times the wavelengthintervals (100 GHz) of the original wavelength multiplexed opticalsignal, and are input to band split filters 14-1 to 14-4.

[0122] The band split filters 14-1 to 14-4 respectively split the inputoptical signals into n bands. Then, optical signals of identicalwavelength bands are combined by interleavers 21 a-1 to 21 b-n and 22-1to 22-n in stages, and fine adjustment for a first order dispersioncompensation is made for the respective bands by DCFs15. After the fineadjustment is made, the optical signals are input to optical DEMUXs 16,which demultiplex the signals into respective channels. The signals arethen received by optical receivers #1 to #40.

[0123]FIG. 16 exemplifies a fifth configuration according to thepreferred embodiment of the present invention.

[0124] In this configuration example, after compensation is madesimultaneously for all of channels by a DCF1, optical signals are splitinto n wavelength bands by band split filters, fine adjustment is madefor the respective wavelength bands by fixed dispersion compensators,and variable dispersion compensators are arranged after the opticalsignals are coupled by band couple filters.

[0125] An optical signal propagated over a fiber transmission line 10 isamplified by an optical pre-amplifier 11, and at the same time, itsdispersion slope is compensated by the DCF1. Then, the wavelengthmultiplexed optical signal is sampled by an interleaver 13, and opticalsignals of 200-GHz intervals are input to band split filters 14 a-1 and14 a-2. The band split filters 14 a-1 and 14 a-2 respectively split theoptical signals into n bands, and input the signals to DCFs15 for therespective bands. Then, the DCFs15 make fine adjustment for a firstorder dispersion compensation. The optical signals for which the fineadjustment is made are input to band couple filters 14 b-1 and 14 b-2,which couple the respective bands and input the coupled signals tovariable dispersion compensators 12-1 and 12-2. The variable dispersioncompensators 12-1 and 12-2 compensate for chromatic dispersion. Theoptical signals are demultiplexed into respective channels by opticalDEMUXs 16, and received by optical receivers #1 to #40.

[0126]FIG. 17 exemplifies a sixth configuration according to thepreferred embodiment of the present invention.

[0127] In contrast to FIG. 16, FIG. 17 shows the configuration wherewavelength intervals are separated into 400-GHz (approximately 3.2 nm)intervals by using two-stage interleavers, optical signals are splitinto n wavelength bands by band split filters, fine adjustment is madefor each of the wavelength bands by fixed dispersion compensators, andvariable dispersion compensators are arranged after the wavelength bandsignals are coupled by band couple filters. Although the number ofvariable dispersion compensators increases, requirements fornon-transmission wavelength bands of the band split filters are relaxed.

[0128] An optical signal propagated over a fiber transmission line 10 isamplified by an optical pre-amplifier 11, and at the same time, itsdispersion slope is compensated by a DCF1. Then, the optical signal issampled by an interleaver 13, and separated into odd- and even-numberedchannels of 200-GHz intervals. The respective odd- and even-numberedchannels thus separated are further separated into even- andodd-numbered channels by interleavers 20-1 and 20-2, and input to bandsplit filters 14 a-1 to 14 a-4.

[0129] The band split filters 14 a-1 to 14 a-4 split the respectiveoptical signals into n bands, and input the optical signals of therespective bands to DCFs for a first order dispersion compensation fineadjustment 15, which make fine adjustment for dispersion compensation.Then, the optical signals for which the fine adjustment is made arecoupled by band couple filters 14 b-1 to 14 b-4. Dispersion compensationis then made for the optical signals by variable dispersion compensators12-1 to 12-4. Outputs of the variable dispersion compensators 12-1 to12-4 are demultiplexed into respective channels by optical DEMUXs 16,and received by optical receivers #1 to #40.

[0130]FIG. 18 explains a chromatic dispersion compensation methodaccording to another preferred embodiment of the present invention.

[0131] This figure shows another dispersion compensation method in thecase of a residual dispersion characteristic of (a) a transmission lineand (b) a DCF1 (for compensation collectively for all of channels),which is similar to that shown in FIG. 11. A plurality of (four in thisfigure) dispersion compensation fibers (fixed dispersion compensators)are arranged, and fine adjustment is made for their lengths, so that adispersion slope of the transmission line, the DCF1 and the DCFs2 (a, b,. . . ) is canceled. Furthermore, residual chromatic dispersion iscollectively compensated by arranging variable dispersion compensatorsfor respective wavelength bands, thereby reducing the residualdispersion of all of channels to a smaller value in comparison with thecase shown in FIG. 11. Because the fine adjustment is made for thedispersion slope with the fixed dispersion compensators in a similarmanner as in FIG. 11, the dispersion and the dispersion slopecharacteristic of a transmission line must be measured beforehand. Ifthe dispersion of the transmission line changes with time due to atemperature change thereafter, the chromatic dispersion amounts of allof the channels vary in the same direction. Therefore, the compensationstate can be maintained by making a concurrently used variabledispersion compensator variable.

[0132] According to the preferred embodiments of the present invention,dispersion compensation for all of channels can be effectively made atlow cost and in less size in a wavelength multiplexing transmissionsystem, even if residual dispersion derived from the wavelengthcharacteristic of a dispersion slope of a transmission line and a DCFoccurs, or if variations of chromatic dispersion and a dispersion slopeare large. As a result, a long-haul transmission can be implemented.

[0133] The following configuration example is an example ofconfiguration that can implement not only the principle shown in FIG. 18but also the principle shown in FIG. 11.

[0134]FIG. 19 exemplifies a seventh configuration according to anotherpreferred embodiment of the present invention.

[0135] This figure shows the configuration where after compensation ismade collectively for all of channels by a DCF1, wavelength intervalsare separated into 200-GHz (approximately 1.6 nm) intervals, the opticalsignals are split into n wavelength bands by band split filters, andfine adjustment is made for the respective wavelength bands by variabledispersion compensators. Variable dispersion compensators the number ofwhich is double the number of wavelength bands are required. A fixeddispersion compensator for making fine adjustment for dispersion slopecompensation may be arranged to make compensation for chromaticdispersion and a dispersion slope, which is explained with reference toFIG. 18, with high accuracy.

[0136] A wavelength multiplexed optical signal propagated over a fibertransmission line 10 is amplified by an optical pre-amplifier 11, and atthe same time, its dispersion slope is compensated by a DCF1. Then, theoptical signal is separated into odd- and even-numbered channels by aninterleaver 13, and the separated optical signals are respectively inputto band split filters 14-1 and 14-2. The band split filters 14-1 and14-2 split the wavelength bands into n bands, and output opticalsignals. DCFs30 are used to make fine adjustment for dispersion slopecompensation. However, they may not be arranged if dispersioncompensation with high accuracy is not demanded on a receiving side. Thereason why the DCFs30 are enclosed by brackets is that it is desirableto arrange the DCFs30, but it does not matter unless they are arranged.

[0137] After the optical signals pass through the DCFs30, dispersioncompensation is made for their respective bands by variable dispersioncompensators 12 a-1 to 12 a-n, and 12 b-1 to 12 b-n. The optical signalsare then transmitted to optical DEMUXs 16, which demultiplex the opticalsignals into respective channels. Optical signals of the respectivechannels are received by optical receivers #1 to #40.

[0138]FIG. 20 exemplifies an eighth configuration of the preferredembodiment according to the present invention.

[0139] In contrast to FIG. 19, FIG. 20 shows the configuration wherewavelength intervals are separated into 400-GHz (approximately 3.2 nm)intervals by using two-stage interleavers, the optical signals are thensplit into n wavelength bands by band split filters, and fine adjustmentis made for the respective wavelength bands by variable dispersioncompensators. Variable dispersion compensators the number of which isfour times the number of wavelength bands are required. A fixeddispersion compensator for making fine adjustment for dispersion slopecompensation may be arranged to make the compensation for chromaticdispersion and a dispersion slope, which is explained with reference toFIG. 18, with high accuracy.

[0140] An optical signal propagated over a fiber transmission line 10 isamplified by an optical pre-amplifier 11, and at the same time, itsdispersion slope is compensated by a DCF1. Then, the optical signal isseparated into odd- and even-numbered channels by an interleaver 13. Theoptical signals are further sampled and separated by interleavers 20-1and 20-2 in a succeeding stage, and input to band split filters 14-1 to14-4. The band split filters 14-1 to 14-4 split the respective inputoptical signals into n bands. Fine adjustment is made for dispersionslope compensation for the split signals by arbitrarily arranged DCFs30,and dispersion compensation is made for the signals by variabledispersion compensators 12 a-1 to 12 d-n. Then, the optical signals areinput to optical DEMUXs 16, which demultiplex the signals intorespective channels. The optical signals are then received by opticalreceivers #1 to #40.

[0141]FIG. 21 exemplifies a ninth configuration of the preferredembodiment according to the present invention.

[0142] In contrast to FIG. 20, FIG. 21 shows the configuration wherefine adjustment is made by variable dispersion compensators aftersignals of the same wavelength band are combined by an interleaver. Withthis configuration, the number of variable dispersion compensators canbe reduced. The number of variable dispersion compensators becomes equalto the number of wavelength bands in this example.

[0143] An optical signal propagated over a fiber transmission line 10 isamplified by an optical pre-amplifier 11, and at the same time, itsdispersion slope is compensated by a DCF1. Then, the optical signal isseparated into odd- and even-numbered channels by an interleaver 13, andthe optical signals are respectively input to band split filters 14-1and 14-2. The band split filters 14-1 and 14-2 split the bands of theoptical signals into n bands, and optical signals of the same band amongthe bands split by the band split filters 14-1 and 14-2 are combined byinterleavers 21-1 to 21-n. Fine adjustment is made for dispersion slopecompensation for the respective bands by arbitrarily arranged DCFs30.Then, dispersion compensation is made by variable dispersioncompensators 12-1 to 12-n, and the optical signals are demultiplexedinto respective channels by optical DEMUXs 16, and received by opticalreceivers #1 to #40.

[0144]FIG. 22 exemplifies a tenth configuration of the preferredembodiment according to the present invention.

[0145] In contrast to FIG. 21, FIG. 22 shows the configuration wherefine adjustment is made by variable dispersion compensators aftersignals of the same wavelength band are combined by an interleaversubsequent to band split filters. With this configuration, the number ofvariable dispersion compensators can be reduced. In this example,200-GHz variable dispersion compensators the number of which is doublethe number of wavelength bands are required. Or, 100-GHz variabledispersion compensators the number of which is equal to the number ofwavelength bands may be arranged after signals of the same wavelengthbands are further combined into 100-GHz signals by interleavers.

[0146] An optical signal propagated over a fiber transmission line isamplified by an optical pre-amplifier 11, and at the same time, itsdispersion slope is compensated by a DCF1. Then, the optical signal isseparated into odd- and even-numbered channels by an interleaver 13. Thesignals are further sampled and separated by interleavers 20-1 and 20-2,and respectively input to band split filters 14-1 to 14-4. The bandsplit filters 14-1 to 14-4 separate the input optical signals into nbands. Next, optical signals of the same band are combined byinterleavers 21 a-1 to 21 b-n, and fine adjustment is made fordispersion slope compensation for the respective bands by DCFs30. Then,dispersion compensation is made for the signals by variable dispersioncompensators 12 a-1 to 12 b-n. The optical signals are demultiplexedinto respective channels by optical DEMUXs 16, and received by opticalreceivers #1 to #40.

[0147]FIG. 23 exemplifies an eleventh configuration of the preferredembodiment according to the present invention.

[0148] This figure shows the configuration where compensation is madesimultaneously for all of channels by a DCF1 and a variable dispersioncompensator for 100-GHz intervals, all of the channels are thendemultiplexed by an optical DEMUX, and fine adjustment is made for therespective channels by fixed dispersion compensators.

[0149] An optical signal propagated over a fiber transmission line 10 isamplified by an optical pre-amplifier 11, and at the same time, itsdispersion slope is compensated by a DCF1. Then, chromatic dispersioncompensation is made for the signal by a variable dispersion compensator12. The optical signal for which the chromatic dispersion compensationis made is demultiplexed into respective channels by an optical DEMUX16. Then, fine adjustment for a first order dispersion is made for therespective channels by DCFs15, and the signals are received by opticalreceivers #1 to #40.

[0150]FIG. 24 exemplifies a twelfth configuration of the preferredembodiment according to the present invention.

[0151] In contrast to FIG. 19, this figure shows the configuration wherevariable dispersion compensators for 200-GHz intervals are arrangedafter wavelength intervals are separated into 200-GHz intervals.

[0152] An optical signal propagated over a fiber transmission line 10 isamplified by an optical pre-amplifier 11, and at the same time, itsdispersion slope is compensated by a DCF1. Then, the optical signal issampled and separated into odd- and even-numbered channels by aninterleaver 13. Dispersion compensation is then made for the respectiveoptical signals by variable dispersion compensators 12-1 and 12-2, andthe signals are demultiplexed into respective channels by optical DEMUXs16. Fine adjustment for a first order dispersion is then made for theoptical signals of the respective channels by DCFs15, and the signalsare received by optical receivers #1 to #40.

[0153] A dispersion compensating apparatus in an inline amplifier isexplained next.

[0154] As a solution to the above described (2) incompleteness of aslope compensation rate of a DCF, (3) manufacturing variations of aslope coefficient of a transmission line/DCF, and (5) influence of thewavelength dependency of a chromatic dispersion slope, a method makingcompensation by splitting a wavelength band into a plurality of bands isalso used.

[0155]FIGS. 25A and 25B show the principle configurations in the casewhere a preferred embodiment according to the present invention isapplied to an inline amplifier. FIGS. 26 and 27 explain the principle ofa dispersion compensation method according to the preferred embodimentof the present invention.

[0156]FIG. 25A shows the fundamental configuration (the case where fixeddispersion compensators are applied) of compensation made by splitting awavelength band into four bands. After a dispersion compensator DCF1(dispersion compensation amount d₁) is arranged in common to all ofchannels of a wavelength multiplexed signal, the signal is split intofour wavelength bands (Λ₁, Λ₂, Λ₃, and Λ₄ from a short wavelength side)by a band split filter 20-1. DCF2 ₁, DCF2 ₂, DCF2 ₃, and DCF2 ₄, whichare intended to compensate for residual dispersion differences among thebands, are arranged in the respective bands. In each span or a pluralityof spans, adjustment is made so that 100-percent dispersion compensationis made in central wavelengths of the respective wavelength bands by thefixed dispersion compensators DCF2 ₁, DCF2 ₂, DCF2 ₃, and DCF2 ₄ in thecase of the residual dispersion characteristics of (a) a transmissionline and (b) a DCF1 shown in FIG. 26. As a result, the residualdispersions of all of the channels can be reduced to small values (d).Not only a dispersion compensation fiber but also various types ofdevices such as a chirped fiber Bragg grating (CFBG), a VIPA dispersioncompensator, etc. are applicable as a dispersion compensator. FIG. 25Bshows the configuration where not a dispersion compensator DCF1 formaking compensation collectively for all of channels, but onlydispersion compensators for making adjustment for bands are applied. Itseffect is similar to that in the case of FIG. 25A. However, dispersioncompensators whose dispersion compensation amounts are large as shown inFIG. 27 must be applied as the fixed dispersion compensators DCF2 ₁,DCF2 ₂, DCF2 ₃, and DCF2 ₄.

[0157] The bands of the optical signals for which the dispersioncompensation is made are coupled by a band split filter 20-2 (that alsofunctions as an element for coupling bands because of the reversibilityof light propagation, which is a characteristic of an optical element,although its name is band split), and the signal of the coupled bands isoutput.

[0158]FIG. 28 shows the configurations where the fixed dispersioncompensators for the respective wavelength bands are replaced byvariable dispersion compensators in correspondence with FIGS. 25A and25B.

[0159] How to set a dispersion compensation amount is the same as thosein FIG. 26 (the case of the configuration shown in FIG. 28A) and FIG. 27(the case of the configuration shown in FIG. 28B). However, since thedispersion compensation amounts are variable, dispersion compensationwith higher accuracy can be made, and besides, a dispersion amount thatchanges with time due to a temperature change in a transmission line, orthe like can be coped with.

[0160] Namely, in FIG. 28A, after dispersion compensation is made for aninput optical signal by a chromatic dispersion compensation fiber DCF1,the signal is split into respective bands Λ₁ to Λ₄ by a band splitfilter 20-1, and input to a band split filter 20-2. The band splitfilter 20-2 couples optical signals, which are split into the respectivebands and input, and outputs the coupled signal as an optical signal ofone band. FIG. 28B shows the configuration where a chromatic dispersioncompensation fiber DCF1 that makes dispersion compensation collectivelyis omitted from the configuration shown in FIG. 28A.

[0161] According to the preferred embodiments of the present invention,dispersion compensation can be effectively made for all of channels atlow cost and in less size in a dense wavelength multiplexingtransmission system, even if manufacturing variations of chromaticdispersion/dispersion slope of a transmission line and a DCF are large,if slope compensation ratio of a DCF is low, or if residual dispersionderived from fourth order dispersion occurs. As a result, a long-haultransmission can be implemented.

[0162]FIGS. 29A to 33B exemplify specific configurations of an inlineamplifier in the case of wavelength band split compensation.

[0163] In FIG. 29A, a pump light source 25 for distributed Ramanamplification is arranged, and an EDFA 26 is arranged in the next stage,so that an optical signal is amplified. After dispersion compensation issimultaneously made by a chromatic dispersion compensation fiber DCF1, awavelength band is split into two bands Λ₁ and Λ₂ by a band split filter20-1. Dispersion compensation is further made for one of the bands by achromatic dispersion compensation fiber DCF2. For the band Λ₁, a(variable or fixed) optical attenuator 27 for providing a loss, which isalmost equal to an optical loss produced by the chromatic dispersioncompensation fiber DCF2, is arranged depending on need. After opticalsignals of the respective bands are coupled by a band split filter 20-2,the coupled signal is output, amplified by an EDFA 28, and output to atransmission line.

[0164]FIG. 29B shows the configuration where a wavelength band is splitinto n bands. The fundamental configuration is the same as that shown inFIG. 29A. Accordingly, the same constituent elements are denoted withthe same reference numerals, and an explanation about the configurationis omitted.

[0165]FIGS. 29A and 29B exemplify the configurations where the inlineamplifier is configured by the two-stage erbium-doped fiber opticalamplifiers (EDFAs). Distributed Raman amplification (DRA) for improvingan optical signal-to-noise ratio (OSNR) is made as occasion demands.FIG. 29A shows the configuration where a wavelength band is split intotwo bands. In this figure, a wavelength band is split into a blue band(Λ₁) on a short wavelength side and a red band (Λ₂) on a long wavelengthside by using the band split filter. The dispersion compensator (DCF1)arranged in the stage preceding the band split filter is set so thatchromatic dispersion is optimally compensated in a central channel ofthe blue band (Λ₁). Therefore, a dispersion compensator is not arrangedon an optical path of the blue band (Λ₁). However, the variable or fixedoptical attenuator for providing an optical loss that is almost equal toa dispersion compensator DCF2 is inserted on the optical path of theblue band depending on need. Assume that the dispersion slope of thetransmission line fiber is S (ps/nm²/km), the slope compensation ratioof the DCF is β (0 to 1), the wavelength difference between the centralchannels of the blue band (Λ₁) and the red band (Λ₂) is ΔΛ (nm), and thetransmission line length per span is L (km). In this case, the residualdispersion difference between the central channels of the red band (Λ₁)and the blue band (Λ₂) per span becomes S·(1−β)·ΔΛ·L. Accordingly, thedispersion compensation amount of the DCF for making adjustment may beset as follows: Δd=−S·(1−β)·ΔΛ·L

[0166]FIG. 29B shows the further expanded configuration where awavelength band is split into n bands. The wavelength band is split inton bands from the short wavelength side to the long wavelength side, andthe dispersion compensators for making adjustment, which are intended tooptimize the dispersion compensation amounts in the respective signalbands after being split, DCF2 ₁, DCF2 ₂, . . . , DCF2 _(n−1), and DCF2_(n) are arranged. The dispersion compensation amounts of the respectivedispersion compensators are set to 0, Δd, . . . , (n−2)·Δd, (n−1)·Δd.Similar to FIG. 29A, the value of Δd may be set as follows:Δd=−S·(1−β)·ΔΛ·L (ΔΛ is the wavelength difference between the centralchannels of adjacent wavelength bands). In both of FIGS. 29A and 29B, avariable or fixed optical attenuator for compensating for a lossdifference between wavelength bands is arranged depending on need.

[0167]FIGS. 30A and 30B show the configurations where the fixeddispersion compensators for the respective wavelength bands in FIGS. 29Aand 29B are replaced by variable dispersion compensators. Although howto set a dispersion compensation amount is the same as those in theconfigurations shown in FIGS. 29A and 29B, the dispersion compensationamount is variable. Accordingly, there are advantages that dispersioncompensation with higher accuracy can be implemented, and that thedispersion amount that changes with time due to a temperature change ina transmission line can be coped with.

[0168] In FIG. 30A, after distributed Raman amplification (amplificationby a pump light source 25), and amplification by an EDFA 26 are made,dispersion compensation for a blue band (Λ₁) is optimized by a DCF1.Then, a wavelength band is split by a band split filter 20-1. Dispersioncompensation for a red band (Λ₂) is optimized by a variable dispersioncompensator 1. An optical attenuator on the path of the blue bandprovides a loss that is almost equal to the loss of the variabledispersion compensator 1 to the optical signal of the blue band. Thisprevents a difference between the optical intensities of the opticalsignals of the blue and the red bands from occurring, when the opticalsignals are coupled by a band split filter. Then, the bands are coupledand output by a band split filter 20-2, amplified by an EDFA 28, andoutput to a transmission line.

[0169] In FIG. 30B, the DCFs shown in FIG. 29B are replaced by variabledispersion compensators. Since a dispersion value that a variabledispersion compensator 1 arranged for the band Λ₁ can compensate for isvariable, a DCF1 in the stage preceding the band split filter may bearranged depending on need.

[0170] The configuration shown in FIG. 30B is similar to that shown inFIG. 30A except that the number of split bands is n. Therefore, the sameconstituent elements as those shown in FIG. 30A are denoted with thesame reference numerals, and their explanations are omitted.

[0171] In FIGS. 31A and 31B, the same constituent elements as those ofthe above described configuration examples are denoted with the samereference numerals, and their explanations are omitted. FIGS. 31A and31B exemplify the configurations where an inline amplifier is composedof a dispersion compensating fiber Raman amplification unit 30 (DCFRA:So called hereinafter is an amplification unit that makes Ramanamplification by using a DCF as an amplification medium. The DCFRA makesoptical amplification and dispersion compensation at the same time), andan EDFA 28 in a later stage. How to split a wavelength band signal, andhow to arrange a dispersion compensation fiber for making adjustment arethe same as those in FIGS. 29A and 29B. The DCFRA 30 is located before aband split filter, and adjusts the power and the wavelength of pumpedlight so as to provide a gain to all of wavelength bands (Λ₁, Λ₂ . . .Λ_(n)). The number of stages of DCFRAs 30 may be 1, 2 or more accordingto a required gain as shown in FIGS. 31A and 31B. In both of FIGS. 31Aand 31B, a variable or fixed optical attenuator for compensating for aloss difference between wavelength bands is arranged depending on need.Because the dispersion compensation of the band Λ₁ is made in the stagebefore the band split filter as described above, a DCF for this band maynot be particularly arranged, and arranged if necessary from a designviewpoint.

[0172] In FIGS. 32A and 32B, the same constituent elements as thoseshown in FIGS. 31A and 31B are denoted with the same reference numerals,and their explanations are omitted. FIGS. 32A and 32B show theconfigurations where the fixed dispersion compensators for therespective wavelength bands in FIGS. 31A and 31B are replaced byvariable dispersion compensators. How to set a dispersion compensationamount is the same as those in the configurations shown in FIGS. 31A and31B. However, the dispersion compensation amount is variable. Therefore,dispersion compensation with higher accuracy can be implemented, andbesides, the dispersion amount that changes with time due to atemperature change in a transmission line can be coped with.

[0173] In FIG. 32A, dispersion compensation for a band Λ₁ is made by aDCF1 in an earlier stage or along with a DCF2 depending on need.Therefore, only a variable or fixed optical attenuator is arranged.

[0174] In FIG. 32B, a variable dispersion compensator 1 is arranged onthe path of the band Λ₁. Therefore, if amplification of an opticalsignal is sufficient, DCF1 and DCF2 may be omitted.

[0175] In FIGS. 33A and 33B, the same constituent elements as thoseshown in FIGS. 31A and 31B are denoted with the same reference numerals,and their explanations are omitted. FIGS. 33A and 33B exemplify theconfigurations where DCFRAs are arranged within a band splitcompensation unit, or arranged before and within the band splitcompensation unit. In FIG. 33A, a wavelength band is split into a blueband (Λ₁) on a short wavelength side, and a red band (Λ₂) on a longwavelength band by using a band split filter. Dispersion compensatorsDCF2 ₁ and DCF2 ₂, which are arranged within the band split compensationunit, respectively make adjustment so that sums d₁+d₂ and d₁+d₂+Δd oftheir dispersion compensation amounts become optimal compensationamounts in central channels of the blue band (Λ₁) and the red band (Λ₂).Furthermore, their lengths are set to obtain sufficient Ramanamplification gains on the compensators DCF2 ₁ and DCF2 ₂ themselves.Similar to FIG. 27, the dispersion compensation amount of the DCF may beset as follows: Δd=−S·(1−β)·ΔΛ·L. Additionally, Raman amplificationgains of the DCF2 ₁ and CDF2 ₂ can be adjusted so that the opticallevels of wavelength bands become equal. Namely, by adjusting a Ramanamplification gain, a function similar to that of an optical attenuatorcan be implemented.

[0176]FIG. 33B shows a further expanded configuration where a wavelengthband is split into n bands. A wavelength band is split into n bands fromthe short wavelength side to the long wavelength side, and dispersioncompensators for making adjustment DCF2 ₁, DCF2 ₂, . . . DCF2 _(n−1),DCF2 _(n), which are intended to optimize dispersion compensationamounts for the respective signal bands after being split, are arranged.Dispersion compensation amounts of the respective dispersioncompensators are set as follows: d₂, d₂+Δd, . . . d₂+(n−2)·Δd,d₂+(n−1)·Δd. The value of Δd may be set in a similar manner as in FIG.33A: Δd=−S·(1−β)·ΔΛ·L (ΔΛ is a wavelength difference between the centralchannels of adjacent wavelength bands). Furthermore, Raman amplificationgains of DCF2 ₁, DCF2 ₂, . . . , DCF2 _(n) are adjusted so that theoptical levels of wavelength bands become equal.

[0177] FIGS. 34 to 36 exemplify the configurations of a system usinginline amplifiers that make band split compensation shown in FIGS. 29 to33.

[0178] These figures show the examples of a 6-span transmission. Asimilar configuration can be implemented also for spans the number ofwhich is different. Additionally, these figures show the examples in thecase where a wavelength band is split into two bands. However, a similarconfiguration can be implemented also when a wavelength band is splitinto a larger number of bands.

[0179] In FIG. 34, band split compensation is made by each of inlineamplifiers 35 according to the above described preferred embodiments ofthe present invention, and a difference of Δd between the dispersioncompensation amounts of split bands is provided. Optical signals ofrespective wavelengths, which are output from transmitters Tx #1 to #n,are coupled by an optical coupler 40, and output to a transmission line.Over the transmission line, the optical signal is relayed by the inlineamplifiers 35, and split into the respective wavelengths by an opticalcoupler 41. The optical signals then pass through arbitrarily arrangedvariable dispersion compensators 42, and are received by receivers Rx #1to #n.

[0180] In FIGS. 35 and 36, the same constituent elements as those shownin FIG. 34 are denoted with the same reference numerals, and theirexplanations are omitted. In FIG. 35, band split compensation is madeevery 2 spans, and a difference of 2Δd between the dispersioncompensation amounts of split bands is provided. In FIG. 36, band splitcompensation is made every 3 spans, and a difference of 3Δd between thedispersion compensation amounts of split bands is provided. As theconfiguration of every 3 spans in FIG. 36 gets closer to that of every 1span in FIG. 34, residual dispersion in an inline amplifier is reducedto a smaller value. Therefore, wavelength degradation caused bychromatic dispersion and non-linear effects of a fiber is suppressed.However, since the number of split compensation units becomes large, thecost and the size increase, which is disadvantageous from the viewpointof securing an optical signal-to-noise ratio (OSNR). Actual arrangementlocations of band split units must be determined in consideration ofsuch a trade-off in an entire system.

[0181] A preferred embodiment for determining how to set a dispersioncompensation amount in an inline section is explained below.

[0182] For the above described (1) variations of the length of atransmission line and (2) manufacturing variations of a chromaticdispersion coefficient, a chromatic dispersion amount (of all ofsections or each inline section) of an installed fiber transmission linemust be actually measured, and a dispersion compensating fiber having achromatic dispersion amount that suits the measured amount must beinstalled. However, since there is the problem of (2) manufacturingvariations of a dispersion slope, strict dispersion compensation cannotbe made for other channels even if chromatic dispersion is strictlycompensated for one channel (such as a central channel). To make strictdispersion compensation, a method securing as large dispersion toleranceas possible in each span, and making strict dispersion compensation forall of channels at a receiver side is effective.

[0183]FIGS. 37A to 37C show a Q penalty (a degradation amount of a Qvalue) against residual dispersion (a total dispersion amount of atransmission line and a dispersion compensator) in the case of an inlinedispersion compensation ratio D_(DCL)=100 percent and 114 percent in a600-km SMF transmission (100 km×6 spans).

[0184] Here, the Q penalty is a difference between a back-to-back valueof a Q value and the Q value after being transmitted over a transmissionline. The Q value is a value obtained by dividing a sum of a standarddeviation of a sample distribution on the side of “1” and that on theside of “0” by the amplitude of signals at the centers of the sampledistributions between the sides of “1” and “0”, when an eye pattern isobtained by converting an optical signal waveform into an electricsignal, and a cross section of the eye is vertically taken at the centerof the eye.

[0185]FIG. 37A plots the Q penalty against a total residual dispersionvalue. FIG. 37B shows an eye pattern when residual dispersion is madezero at a receiving end in the case where an inline dispersioncompensation amount is set to compensate for a propagation dispersionamount by 100 percent. FIG. 37C shows an eye pattern in the case wherethe inline dispersion compensation ratio is set to compensate for thepropagation dispersion amount by 114 percent.

[0186] As is evident from FIGS. 37A to 37C, wavelength degradation andthe Q penalty in the case of the 114-percent over-compensation aresmaller than those in the case of the 100-percent dispersioncompensation (complete wavelength deformation is more advantageous)However, the residual dispersion must be strictly made zero by adjustingthe dispersion compensation amount (D_(DCR)) at a receiving end. Notethat, however, D_(DCT) may be optimized to a different value dependingon a difference in a transmission condition (fiber type, transmissiondistance, bit rate, etc.).

[0187]FIG. 38 shows a Q penalty characteristic against an inlinedispersion compensation residual amount (the dispersion amount of atransmission line per span +the residual dispersion amount of an inlineDCF) in the case where the residual dispersion is made zero by adjustingthe dispersion compensation amount (D_(DCR)) at the receiving end ineach case in a 600-km SMF transmission (100 km×6 spans).

[0188] It can be verified that over-compensation (an inline dispersioncompensation residual amount is negative) makes the penalty smaller foran inline DCF. Furthermore, it is proved that a tolerance (approximately400 ps/nm when a 1.5 dB penalty is allowed), which is considerablylarger than the dispersion tolerance (approximately 70 ps/nm) of a40-Gb/s signal, can be secured.

[0189] Here, a dispersion shift amount AD is given by the followingequation.

ΔD (ps/nm)=(dispersion amount per span (ps/nm))×(1−dispersioncompensation ratio)

[0190] Here, dispersion compensation rate=(percentage of dispersioncompensation ratio)/100. In the case shown in FIG. 38, (dispersionamount per span (ps/nm))=(17 (ps/nm/km))×(100 (km) (per span))=(1700(ps/nm)).

[0191] Especially, the Q penalty is good in the neighborhood of −200ps/nm, where the inline dispersion compensation amount corresponds tothe approximately 114-percent compensation, and an especially goodcharacteristic is obtained in a range from approximately 105-percentcompensation to approximately 120-percent compensation.

[0192] Accordingly, according to the preferred embodiments of thepresent invention, dispersion compensation for all of channels can beeffectively made at low cost and in less size in an ultrahigh-speedwavelength multiplexing transmission system, even if manufacturingvariations of chromatic dispersion/dispersion slope of a transmissionline and a DCF are large, if a slope compensation rate of a CCF is low,or if residual dispersion derived from the wavelength dependency of adispersion slope occurs. As a result, a long-haul transmission can beimplemented.

[0193]FIG. 39 exemplifies a first configuration corresponding to apreferred embodiment for optimizing an inline dispersion compensationamount.

[0194] In this figure, the same constituent elements as those shown inFIG. 34 are denoted with the same reference numerals, and theirexplanations are omitted.

[0195] If dispersion amounts in central channels in respective inlinerelay sections are actually measured as D₁, D₂, . . . , D_(n−1), D_(n),dispersion compensation amounts of inline amplifiers ILA1, ILA2, andILA(n−1) are set as follows: d_(DCL1)=−(1+γ)·D₁, d_(DCL2)=−(1+γ)·D₂, . .. , d_(DCL(n−1))=−(1+γ)·D_(n−1)·γ is an over-compensation rate of adispersion compensation amount, and typically 0.10 to 0.15 (10 to 15percent). At a transmitting end, a dispersion compensation (dispersioncompensation in an optical post-amplifier) DCT (dispersion compensationamount d_(DCT)) for improving a transmission characteristic can possiblybe arranged (d_(DCT)=0 as a typical value (not arranged)). To set theresidual dispersion (the total dispersion amount of the transmissionline and the dispersion compensator) to D_(RD) (zero as a typical value)by making up for the over-compensation of the dispersion inlineamplifier, the dispersion compensation amount at the receiver side of areception dispersion compensator DCR of an optical pre-amplifier is setto γ(D₁+D₂+ . . . +D_(n−1))−d_(DCT)−D_(n)+D_(RD).

[0196] If the dispersion amount of spans are equal (D₁=D₂= . . .=D_(n−1)=D_(n)=D), ((n−1)γ−1)D−d_(DCT)+D_(RD) is obtained. Furthermore,a variable dispersion compensator for adjusting various types ofdispersion variations or changes with time can possibly be arranged foreach channel or all of channels.

[0197]FIG. 40 shows one example of a further specific configuration.

[0198] In this figure, the same constituent elements as those shown inFIG. 39 are denoted with the same reference numerals.

[0199] In each of inline amplifiers 50, distributed Raman amplification(DRA) and dispersion compensating fiber Raman amplification (DCFRA)using a DCF are made. The number of stages of DCFRA units can possiblybe one or more according to a required gain. In either case, the lengthof each DCF is adjusted so that the sum of dispersion compensationamounts of DCFs becomes equal to the dispersion compensation amountshown in FIG. 39, and a required value or larger of the Ramanamplification gain of each DCF can be secured. At that time, anover-compensation rate γ of the dispersion compensation amount must beset not to be too large so as to secure the length which uses the Ramangain at the receiving end DCF as a required amount.

[0200] A repeater 51 at a receiving end has fundamentally the sameconfiguration as that of the inline amplifier 50. However, an opticalamplifier is an optical pre-amplifier, which makes amplification fordetecting a signal at the receiving end.

[0201]FIG. 41 exemplifies the configuration implemented by combiningband split dispersion compensation and inline over-compensation.

[0202] In this figure, the same constituent elements as those shown inFIG. 40 are denoted with the same reference numerals, and theirexplanations are omitted.

[0203] For respective inline amplifiers 50, dispersion compensationfibers DCL1 to DCL(n−1) for making over-compensation are arranged, andover-compensations of d_(DCL1) to d_(DCLn−1) are made. Furthermore, bandsplit filters are arranged at a stage after the dispersion compensationfibers, which respectively make dispersion compensation for bands.

[0204] A dispersion compensator arranged within an optical pre-amplifierat a receiving end makes dispersion compensation so that residualdispersion becomes zero in the dispersion compensation fiber DCR and theband split dispersion compensator in the stage after the dispersioncompensation fiber. Or, after a wavelength multiplexed optical signalmay be demultiplexed into optical signals of respective wavelengths,fine adjustment may be respectively made for dispersion compensation byarranging variable dispersion compensators.

[0205] According to the present invention, dispersion compensation canbe effectively made at low cost and with high accuracy in a wavelengthmultiplexed optical communication.

What is claimed is:
 1. A chromatic dispersion compensating apparatus foruse in a wavelength-division multiplexed optical transmission system,comprising: a band splitting unit splitting a wavelength-divisionmultiplexed optical signal into a plurality of wavelength bands; and afixed dispersion compensating unit making compensation for residualdispersion for split wavelength-division multiplexed optical signals. 2.A chromatic dispersion compensating apparatus for use in awavelength-division multiplexed optical transmission system, comprising:a band spitting unit splitting a wavelength-division multiplexed opticalsignal into a plurality of wavelength bands; and a variable dispersioncompensating unit making compensation simultaneously for splitwavelength multiplexed optical signals.
 3. The chromatic dispersioncompensating apparatus according to claim 1 or 2, further comprising aninterleaving unit, which is arranged at a stage before said bandspitting unit, separating the wavelength-division multiplexed opticalsignal, and converting the wavelength-division multiplexed opticalsignal into a plurality of optical signals whose channel intervals arewidened.
 4. A chromatic dispersion compensating apparatus for use in awavelength-division multiplexed optical transmission system, comprising:a variable dispersion compensating unit making dispersion compensationsimultaneously for a whole or part of a wavelength multiplexed opticalsignal; an optical splitting unit splitting the wavelength multiplexedoptical signal; and a fixed dispersion compensating unit makingcompensation for residual dispersion of an optical signal of each splitchannel.
 5. The chromatic dispersion compensating apparatus according toclaim 4, wherein said optical splitting unit splits thewavelength-division multiplexed optical signal into a plurality ofwavelength bands.
 6. The chromatic dispersion compensating apparatusaccording to claim 4, wherein said optical splitting unit splits thewavelength multiplexed optical signal into optical signals of respectivechannels.
 7. The chromatic dispersion compensating apparatus accordingto claim 1, 2, or 4 is arranged within a optical receiver.
 8. Thechromatic dispersion compensating apparatus according to claim 1, 2, or4 is arranged within an inline amplifier.
 9. The chromatic dispersioncompensating apparatus according to claim 1, 2, or 4, wherein the inlinedispersion compensation makes 105- to 120-percent compensation forchromatic dispersion that a received wavelength multiplexed opticalsignal undergoes.
 10. A chromatic dispersion compensating apparatus foruse in a wavelength multiplexed optical transmission system, comprising:a band splitting unit splitting a wavelength-division multiplexedoptical signal into a plurality of wavelength bands; and a dispersioncompensating unit reducing a residual dispersion difference betweenbands for respective split wavelength bands.
 11. The chromaticdispersion compensating apparatus according to claim 10, wherein saiddispersion compensating unit is a fixed dispersion compensator.
 12. Thechromatic dispersion compensating apparatus according to claim 10,wherein said dispersion compensating unit is a variable dispersioncompensator.
 13. The chromatic dispersion compensating apparatusaccording to claim 10 is arranged within an inline amplifier.
 14. Thechromatic dispersion compensating apparatus according to claim 10,further comprising a simultaneous dispersion compensating unit makingdispersion compensation simultaneously for an input wavelength-divisionmultiplexed optical signal, wherein said simultaneous dispersioncompensating unit makes inline 105- to 120-percent compensation forchromatic dispersion of each span that a received wavelength-divisionmultiplexed optical signal undergoes.
 15. A wavelength multiplexedoptical transmission system using the chromatic dispersion compensatingapparatus according to claim 10 is a system where an inline amplifierthat includes the chromatic dispersion compensating apparatus and aninline amplifier that does not include the chromatic dispersioncompensating apparatus are mixed.
 16. The chromatic dispersioncompensating apparatus according to claim 13, wherein optical amplifiersare arranged in a stage before or after the chromatic dispersioncompensating apparatus within the inline amplifier.
 17. The chromaticdispersion compensating apparatus according to claim 10, wherein anoptical amplifier, which uses a dispersion compensation fiber as a Ramanamplification medium, is arranged for the respective split wavelengthbands.
 18. A chromatic dispersion compensating apparatus for use in amulti-span optical transmission system, comprising a dispersioncompensating unit making 105- to 120-percent over-compensation for achromatic dispersion amount of each span in each inline amplifier. 19.The chromatic dispersion compensating apparatus according to claim 18,wherein compensation is made so that a total dispersion amount is closeto zero at an optical receiver in the multi-span optical transmissionsystem.
 20. The chromatic dispersion compensating apparatus according toclaim 19, wherein dispersion compensation amounts within each inlineamplifier and at the receiver side are set based on chromatic dispersionamount information of each span of a central channel of awavelength-division multiplexed optical signal.